In optical access networks, it is desirable to provide support for a variety of services. These services may include, fiber-to-the-home (FTTH) services, fiber-to-the-building (FTTB) services, enterprise/business connectivity services, and services for mobile backhaul and front-haul supporting fourth generation (4G) and future fifth generation (5G) wireless communication. To cost-effectively support these diverse applications, optical access networks need to provide high-speed connectivity and wide distance coverage. However, with the increase of optical transmission speed and/or transmission distance, signal degradation becomes more severe and eventually prevents the realization of high-speed wide-coverage optical access. In optical transport networks, there is also an increasing demand to reduce the cost of the optical transceivers by using intensity-modulation and direct-detection (IM/DD). However, IM/DD formats suffer poor transmission performance as compared to coherent-detection formats.